You could force BOINC to run more than one process on each processor (core), but that wouldn't increase productivity; on the contrary, two tasks on one core would take more than double the time to finish. The exception to that would be "hyperthreading", a feature of some (Intel?) processors, but I can't give you more information on that (I don't even know if your computer is hyperthreading or not:-).

So, the only meaningful way to increase the number of processes would be adding hardware to your computer, a GPU (Graphics Processing Unit) for instance.